1. Field of the Invention
This invention relates to a compound type magnetic head and more particularly to a compound type magnetic head having a core and auxiliary cores and capable of corresponding to a high coercive medium.
2 Description of the Prior Art
As well known, in order to sufficiently magnetize a magnetic recording medium of a high coercive medium such as a metal tape and vacuum deposited type, a head core sufficiently high in saturation flux density is required. To meet this requirement, alloy magnetic materials such as sendust, amorphous material have been developed. But the alloy magnetic materials are not free from the problem that the materials are low in inherent resistance, reduced in magnetic permeability by eddy current loss in the video frequency zone and reduced in the reproducing efficiency of a magnetic head.
In order to solve the above described problem of the magnetic head of an alloy magnetic material, a compound type magnetic head consisting of a main core made of an alloy magnetic material and auxiliary cores made of ferrite higher in inherent resistance than the alloy magnetic material has heretofore been proposed. Namely, the main core containing a front gap is formed of an alloy magnetic material and a portion other than the neighborhood of the gap occupying a major part of a magnetic path is formed of ferrite high in inherent resistance. The compound type magnetic head of this type is disclosed, for example in Japanese Utility Model Registration Application No. 83051/1971 (Japanese Utility Model Publication Laid Open to Public Inspection No. 39918/1973).
FIG. 1 is a perspective view of a conventional compound type magnetic head of the type described above.
FIG. 2 is an exploded view of the compound type magnetic head in FIG. 1. The compound magnetic head 10 comprises a main core 11 and two auxiliary cores sandwiching both sides of the main core 11 therebetween. The main core 11 is made of an alloy magnetic material such as sendust, while the other auxiliary cores 12 are made of ferrite. The main core 11 having a winding opening 21 formed therein is formed by depositing a main core half 11a to a main core half 11b with silver solder at deposit portions 22a and 22b. On that side 13 of the main core 11 against which a magnetic tape abuts is formed a head gap 23 made of SiO.sub.2 sputtering. On the other hand, the auxiliary core 12 having a winding opening 21 formed therein is formed by glass depositing auxiliary core halves 12a and 12b at deposit portions 24a and 24b. On those sides of the auxiliary cores 12 against which a magnetic tape abuts is formed a pseudo-gap 26 of a pseudo-gap length l. The conventional compound type magnetic head 10 shown in FIG. 1 is manufactured in the manner that a main core and auxiliary cores of block deposit material (not shown) extending in the direction of track width of a magnetic tape are cut thin and thereafter is ground, laid one over the other, bonded to each other with an organic adhesive agent and fixed to a specified head holder (not shown) and a magnetic tape abutting face 13 is polished as with a polishing tape.
In the conventional compound type magnetic head of the construction described above, the main core 11 made of sendust high in saturation flux density works effectively on the neighborhood of the gap 23 at which saturation of the core is called into question at the time of recording. On the other hand, the auxiliary cores made of ferrite high in magnetic permeability work effectively on a major part of a magnetic path except the pseudo-gap length l at the time of reproduction at which saturation of the core does not come into question. Accordingly, a magnetic head higher in reproducing efficiency is obtained in proportion as the pseudo-gap length l is shorter. The pseudo-gap length l used is in the range of 50 to 100 .mu.m. But because the pseudo-gap 26 is constructed so as to abut against the magnetic tape, the pseudo-gap not only detects a main track signal responsive to the gap 23 of the main core 11 but also detects a signal coming from the track near the main track such that so-called crosstalk is produced. This crosstalk can be reduced to a certain degree by providing azimuth between the gap 23 of the main core 11 and the pseudo-gap 26 of the auxiliary cores 12 or increasing the size of the pseudo-gap length l. But it cannot be said that this can essentially solve the problem of noise being increased by the signal being detected at the edge portion of the pseudo-gap 26.
On the other hand, sendust material of the main core 11 is a material lower in hardness and more liable to wear away than the ferrite constituting the auxiliary cores 12. Accordingly, the main core 11, while making repeated contact with the magnetic tape, is more heavily worn away than the auxiliary cores 12 and is brought into a sunken state, with the result that there is produced a difference in the level of the core surface 13 abutting against the magnetic tape. This amount of sinking is on the order of submicron. Accordingly, in the reproduction of a video signal of the order of a wavelength of 1 .mu.m spacing loss due to this amount of sinking is unignorably large and brings about a great reduction in short wavelength characteristic. Especially, the effects by the difference in level are conspicuous when the pseudo-gap length l measures less than 100 .mu.m. If the pseudo-gap length l is increased, the effects by this level difference can be alleviated to a certain degree, but reproducing efficiency is reduced as described above.
As above, since the conventional compound type magnetic head is of the construction in which the pseudo-gap abuts against the magnetic tape, it is impossible to completely prevent crosstalk, and there is a disadvantage of the difference in the level of the magnetic head pseudo-gap abutting against the magnetic tape bringing about a substantial reduction in short wavelength recording and reproducing characteristics.
On the other hand, a treatise entitled "Newly developed sendust video head for high coercive tape" printed in "IEEE TRANSACTIONS ON MAGNETICS, Vol. Mag-16, No. 5 September 1980" discloses a compound type magnetic head having the main magnetic core of Sendust sandwiched between block materials of nonmagnetic material. But this prior art merely discloses the magnetic head of the construction in which an alloy magnetic material is sandwiched between nonmagnetic materials. Accordingly, the prior art cannot overcome the disadvantage of the alloy magnetic material due to low inherent resistance, namely the disadvantage of a reduction in magnetic permeability due to eddy current loss in the video frequency zone and the resulting reduction in the reproducing efficiency of the magnetic head.